The present embodiments relate to a sheath current filter and a local coil with a sheath current filter.
Magnetic resonance devices (MR devices) for examining objects or patients using magnetic resonance tomography are known, for example, from DE10314215B4.
In nuclear magnetic resonance tomography, high frequency (HF) coils (e.g., local coils) are used to receive alternating magnetic fields. In order to maintain a good signal-to-noise ratio, the geometry and receive profile of HF coils are optimized for different regions of the body. To obtain the highest possible signal-to-noise ratio, the local coils are positioned as closely as possible on the patient's body. Local coils are, for example, embodied as pure receiving coils (e.g., the spins are excited with a separate transmitting coil, the whole-body resonator). For patient monitoring or for importing audio or video signals (e.g., for display for the patient in a head coil), external lines are led to the patient (e.g., into a head coil).
In the transmission phase of the MRT, the E and B fields induce HF currents on the lines of the probes, headphones or visual displays. The HF currents on the supply lines are known as sheath currents. Unwanted HF currents may result in image interference and malfunctions and the endangerment of the patient. Therefore, either the supply lines have very high impedance or chokes. In the case of shielded lines, sheath current filters (SCFs), are inserted to suppress high-frequency currents on conductors. The sheath current filter and chokes represent high impedance for an HF wave and hence suppress the HF current on the line. Although high-impedance lines may be used to receive signals, the high-impedance lines are very susceptible to interference. The lines may not to be high-impedance for the transmission of videos or acoustic signals. In this case, several chokes may be inserted, for example, in each individual line to suppress HF currents. A plurality of individual lines may be combined with a common conductive shield to form one large cable. This line is wound up to form a cylindrical coil and brought to resonance with a capacitor. The high impedance of the parallel resonance prevents the propagation of a sheath current on the outer jacket of the line. Again, a plurality of sheath current filters is connected in series.